Replace removed function pipeworks.string_startswith

[Emojigit]

This fixes the following error:

~/.minetest/mods/technic/technic_cnc/pipeworks.lua:45: attempt to call field 'string_startswith' (a nil value)
stack traceback:
	[C]: in function 'string_startswith'
	~/.minetest/mods/technic/technic_cnc/pipeworks.lua:45: in function 'pipeworks_on_receive_fields'
	~/.minetest/mods/technic/technic_cnc/api.lua:367: in function <~/.minetest/mods/technic/technic_cnc/api.lua:363>

This PR is ready for review.

[OgelGames]

Not sure how I missed that when I searched for it... 🤔

[BuckarooBanzay]

do i understand this correctly: right now the current master branch crashes if you use the technic-cnc machine with pipeworks? 🤔

[S-S-X]

do i understand this correctly: right now the current master branch crashes if you use the technic-cnc machine with pipeworks?

Yes has to crash, pipeworks.string_startswith got removed here mt-mods/pipeworks@223c90e so it isn't available anymore.

Also it would be better this way anyway.

[S-S-X]

Looks good, should fix the issue.

Either of the suggestions can be added, mostly matter of taste. Approval either way. Also works as is without applying suggestions. All of these three options are just matter of taste.